The present invention relates generally to MRI scanners, and, more specifically, to magnetic field generators therein.
A magnetic resonance imaging (MRI) system or scanner is commonly used for precisely determining structure of organic molecules. A target is placed in an imaging volume or zone under a strong magnetic field and analyzed by the absorption and re-emission of radio frequency-electromagnetic radiation by hydrogen or carbon nuclei. The resonant frequency of this absorption and re-emission is a function of the gyromagnetic ratio of the nuclei and the applied magnetic field.
MRI imaging is a derivative of nuclear magnetic resonance (NMR) spectroscopy used by organic chemists to determine organic molecule structure. In NMR spectroscopy, variations in emission intensity as a function of frequency are used to infer variations in the structure of the organic molecule being examined. These frequency variations are due to variations in the local magnetic field caused by variations in the electronic and molecular structure of the organic molecule.
In MRI imaging, variations in emission intensity as a function of frequency are used to generate an image of the target which is typically a selected portion of a human patient. Frequency is used to encode spatial address information. Variations in local magnetic field are created by a pulsed gradient coil system to give a discrete and slightly different field and corresponding frequency for each volume element in the field of view.
The applied magnetic field for NMR spectroscopy is substantially high, and requires a superconducting magnet. The applied magnetic field for MRI imaging is substantially lower and is typically provided by a superconducting magnet, and more recently by permanent magnets with even lower magnetic field strength.
The use of permanent magnets in the magnetic field generators of an MRI scanner substantially reduces the complexity and cost thereof. And, due to advances in improving resolution and image quality of MRI scanners, performance of permanent magnet-based MRI scanners has been improved.
Nevertheless, the relatively high magnetic field strength required for MRI imaging requires a high performance permanent magnet such as rare earth permanent magnets having magnetic energy densities substantially greater than conventional ferrite magnets for example. The typical high performance permanent magnet for MRI scanners is the sintered rare earth neodymium (Nd), iron (Fe), and boron (B) magnet.
The significant magnetic properties of the permanent magnet for an MRI application include the residual magnetic flux density (Br), coercive force (Hc), intrinsic coercive force (Hci), and maximum energy product (BH)max.
The sintered NdFeB rare earth permanent magnet provides high performance for use in various applications such as the MRI magnetic field generator, as well as for use in various portions of a computer including its hard drive and actuation motors. The composition of the permanent magnet and the sequential processes from mine to finished product are currently optimized for NdFeB to obtain the highest energy product (BH)max and the highest intrinsic coercive force Hci.
However, the resulting high performance permanent magnet as used for MRI scanners requires well over a thousand kilograms thereof per scanner which is orders of magnitude greater than the small gram amounts thereof required for a typical computer. Accordingly, the cost of using permanent magnets in an MRI scanner is substantially high which correspondingly limits the practical availability thereof.
The production of permanent magnets for the MRI scanner necessarily begins by initially mining the ore which contains a mixture of various rare earth elements and other miscellaneous elements. The particular rare earth element of interest, such as Nd, must be refined from the basic ore into a substantially pure form greater than about 99%. The rare earth element is then alloyed with separately refined elements such as iron and boron to form an alloy thereof. The alloy in powder form is compacted under pressure in a magnetic field, and heat sintered to form blocks of permanent magnets which are magnetized and assembled in the required configuration for the magnetic field generator of the MRI scanner. The remainder of the scanner is then assembled for cooperating with the permanent magnets.
The resulting cost of the MRI scanner includes in significant part the corresponding high cost to process the rare earth ore for isolating the specific rare earth element followed in turn by alloying the rare earth element with iron and boron to produce the resulting rare earth permanent magnets.
Accordingly, it is desired to reduce the cost of a MRI scanner by reducing the cost of the rare earth permanent magnets therein, and the costs in processing the rare earth elements thereof.
A permanent magnet for an MRI scanner is made by removing extraneous elements from an ore containing rare earth elements to leave elements Pr and Nd therein, and then selectively stripping therefrom a portion of the element Nd as a byproduct to leave an ore residuum including both elements Pr and Nd therein. The residuum is alloyed with a transition metal to form an alloy therewith. The alloy is then formed into a rare earth permanent magnet configured for use in the MRI scanner.